Fc receptors are a group of molecules that bind to the Fc regions of immunoglobulin molecules. The individual molecules recognize single immunoglobulin isotypes, or those of the same group, by recognition domains on the Fc receptor, depending on the recognition domain belonging to an immunoglobulin superfamily. This determines which accessory cells will be driven in an immune response. Fc receptors can be further classified into several subtypes, namely Fcγ receptors which are receptors for IgG (immunoglobulin G), Fcε receptors that bind to the Fc region of IgE, and Fcα receptors that bind to the Fc region of IgA. Each of these receptors have still more detailed classifications, with reported Fcγ receptors including FcγRI, FcγRIIa, FcγRIIb, FcγRIIIa and FcγRIIIb (NPL 1: Takai. T., Jpn. J. Clin. Immunol., 28, 318-326, 2005).
Among the Fcγ receptors, FcγRIIIa are found on the cell surfaces of natural killer cells (NK cells) and macrophages, and they are important receptors involved in the activation of the important human immunomechanism ADCC (antibody-dependent cell-mediated cytotoxicity). The affinity between FcγRIIIa and human IgG has a coupling constant (KA), representing the binding strength, of 107 M or less (NPL 2: J. Galori et al., Eur. J. Immunol., 27, 1928-1932, 1997). The amino acid sequence of human FcγRIIIa (SEQ ID NO: 1) has been published in public databases such as UniProt (Accession number: P08637). Moreover, the functional domain on the structure of human FcγRIIIa, the signal peptide sequence for spanning of the cell membrane, and the position of the transmembrane region have likewise been published. FIG. 1 shows a structural diagram of human FcγRIIIa. The amino acid numbers in FIG. 1 correspond to the amino acid numbers in SEQ ID NO: 1. Specifically, the region from methionine (Met) at position 1 to alanine (Ala) at position 16 of SEQ ID NO: 1 is the signal sequence (5), the region from glycine (Gly) at position 17 to glutamine (Gln) at position 208 is the extracellular domain (EC), the region from valine (Val) at position 209 to valine (Val) at position 229 is the transmembrane region (TM), and the region from lysine (Lys) at position 230 to lysine (Lys) at position 254 is the cytoplasmic (C). It is known that FcγRIIIa binds particularly strongly to IgG1 and IgG3, among the human IgG subclasses from IgG1 to IgG4, while binding weakly to IgG2 and IgG4.
The development of drugs utilizing the specificity of monoclonal antibodies (antibody drugs) has been progressing in recent years. Among the human IgG used in antibody drugs, it is known that ADCC (antibody-dependent cell-mediated cytotoxicity) activity varies due to differences in N-linked sugar chains added to the asparagine residue at position 297 of the Fc region, and it has been reported that ADCC activity is particularly increased with antibodies from which fucose sugar chains have been removed (NPL 3: Shinkawa, T., J. Biol. Chem., 278, 3466-3473, 2003). In other words, the sugar chain structure of an antibody is highly significant for an antibody drug. However, antibody drugs are usually produced using gene recombinant technology with animal cells as the host, and it is difficult to control the sugar chains that are added to antibodies inside the host. Furthermore, much time and effort is required to analyze the sugar chains of produced antibodies.